When a good electrical conductor such as a metal is placed in a high-frequency alternating magnetic field eddy currents are produced in the conductor as a result of electromagnetic induction to cause an "eddy-current loss". If this good conductor is a magnetic material, a hysteresis loss also occurs. As is well known, the high-frequency induction heating apparatus utilizes the heat produced by the eddy-current loss and hysteresis loss.
The so-called "skin effect" produces the eddy currents more concentrated in the surface layer than in the inner portion in the material being heated by induction. The penetration depth of the eddy currents decreases with increasing frequency of the current flowing through the induction heating coil. It is also well known that the eddy-current loss increases with the amount of current flowing through the induction heating coil and with its frequency.
A typical layout of a high-frequency induction heating apparatus is shown schematically in FIG. 1. The heater includes a rectifier 1 for effecting AC to DC conversion, a capacitor for smoothing the resulting DC voltage, an inverter 3 for generating a high-frequency voltage to be supplied to an induction heating coil 4 that is coiled around a material 5 to be heated. The rectifier 1 is necessary only when the high-frequency apparatus receives power from an AC source and, therefore, is not necessary if the apparatus operates on a DC supply. The capacitor 2 is also unnecessary if the ripple voltage resulting from the AC to DC conversion effected in the rectifier 1 can be ignored.
A DC voltage that is obtained either from a three-phase or single-phase power supply after DC-AC conversion in the rectifier 1 and smoothing by the capacitor 2, or from a DC power source, is fed into the high-frequency inverter 3 to produce a high-frequency alternating current that is supplied to the heating coil 4. The resulting eddy currents induced in the material placed within or near the coil produce sufficient thermal energy to heat the material. This is the well-known operating mechanism of a high-frequency induction heating apparatus.
In certain applications of the induction heating technique, such as quenching of steel materials, there exists a need to heat the surface layer and the interior to different temperatures. Conventionally, this need has been met by employing two separate high-frequency induction heaters producing outputs of different voltages and frequencies that are installed side by side and through which a material to be heated is moved such that the surface layer of the material is heated by one unit and the deeper portion of the material is heated by the other unit.
FIG. 2 shows a high-frequency induction heating apparatus that utilizes two induction heater units for the purpose of heating the surface layer and the deeper portion of a material to be heated to different degrees. The first induction heater unit includes a rectifier 11, a capacitor 21, a first high-frequency inverter 31, and a coil 41. The second induction heater unit includes a rectifier 12, a capacitor 22, a second high-frequency inverter 32 that produces an output that is different in power and frequency from the output produced by the first inverter, and a coil 42. The coils 41 and 42 encircle the material 5 to be heated. As the material 5 is moved successively through the coils 41 and 42, it is supplied with different outputs of high-frequency power.
As is apparent from the above explanation, heating with two high-frequency induction heater units requires separate rectifiers and inverters for each unit and hence is expensive.
A less expensive system has been developed and is shown schematically in FIG. 3. In this system, a rectifier 1' and a capacitor 2' are used commonly for two high-frequency inverters 31 and 32. The outputs of an inverter 31 are passed through cross-current suppressing reactors 611 and 612 and supplied to a common coil 4 in which they are superposed on the outputs of another inverter 32 that have been supplied to the coil 4 after passing through cross-current suppressing reactors 621 and 622. This arrangement enables the surface layer and the deeper portion of the material 5 in the coil 4 to be heated to different temperatures because the two power outputs have different frequencies. However, even this system is not very economical since it requires two high-frequency inverters and four cross-current suppressing reactors.